Anion recognition continues to be a major research goal for many supramolecular chemistry groups around the world. As the field matures there is an increasing emphasis on synthetic receptors that operate in aqueous solution. This is because most of the important biomolecular targets such as peptides, nucleotides, phospholipids, and carbohydrates are anionic compounds. However, anion recognition in water is an extremely challenging task for a number of reasons. For a start, anions are strongly hydrated and any complexation process that involves anion dehydration will likely have to pay a large energetic penalty. Common anions, such as halide ions (fluoride, chloride, bromide, and iodide), sulphide, sulphate, chromate, phosphate, oxalate, and nitrite, play vital roles in the environment, industry and biology. Some of these common anions are essential for our life. Anions play an important role in health, food, biomedical and defense industries. Their release in the environment cause hazardous effects to living system by drastically affecting the food and water quality. This necessitates the development of highly sensitive anion sensor materials for monitoring the presence of these toxic pollutants. There is an immense upsurge in last 2-3 decades in designing and synthesizing suitable anion sensors. Classical methods for detection of these hazardous anions which include spectroscopic, electrochemical and chromatographic are very expensive and time consuming. Therefore, in recent years, the investigation on anions sensing methodologies is focused on exploring rapid, versatile, cost-effective and optical detectors.
Vapochromic materials suitable for sensing volatile organic compounds (VOCs) by the alteration of auro- and metallophilic attractions have attracted considerable attention. Whereas these systems often include metallophilic Pt—Pt, Au—Tl, Au—Au, and Au—Ag interactions, complexes with Au—Cu metal centers are generally less common, and such vapochromic complexes are understood to have not been reported.
Vapochromic materials have recently been incorporated in chemical sensor devices. For example, [Au—(PPh2C(CSSAuC6F5)PPh2Me)2][ClO4] has been used in the development of an optical fiber VOC sensor. A vapochromic light emitting diode and a vapochromic photodiode have also been built using tetrakis(p-dodecylphenylisocyano) platinum tetranitroplatinate and bis(cyanide)-bis(p-dodecylphenylisocyanide)platinum(II), respectively.
Anion sensors are broadly classified into two main categories based on their anion binding mode, 1) Binding via non-covalent interactions: anion sensor recognizes or detects anions through donor acceptor non-covalent interactions which include electrostatic, hydrophilic, hydrophobic, hydrogen bonding and π . . . π interactions; 2) Linkage through covalent bond: the analyte directly ligates to the metal based coordination complexes through coordinate bond. Designing non-covalently interacting anion sensing receptors suffer major challenges for their usage in aqueous media due to strong water-anion interaction through H-bonding. Higher energy requirement of receptor binding to hydrated anions also makes it an unfavorable process. Covalently bonded metal based receptors can overcome this problem by strongly interacting with anions through coordinate bond. The binding process can be monitored by visible changes in colour or changes in fluorescence intensity.
Article titled “Anion Recognition and Sensing by a New Macrocyclic Dinuclear Copper (II) Complex: A Selective Receptor for Iodide” by J S Mendy et al. published in Inorg. Chem., 2010, 49 (16), pp 7223-7225 reports a macrocyclic dinuclear copper complex, [Cu2II(1)Br3(H2O)]Br synthesized and characterized by X-ray crystallography, in which the macrocycle is folded to form a bowl-shaped cavity. The sensing ability of the receptor was studied for halides by UV-VIS spectroscopy in water-acetonitrile (1:3 v/v) and water. The article mentioned reported a macrocyclic dinuclear complex [Cu2 II(1)Br3(H2O)]Br, which showed a strong affinity only for iodide anion. However, the media used for binding studies is not completely aqueous but the mixture of aprotic solvent CH3CN and water (3:1). Complete aqueous media (100% water) may inhibit efficient binding with metal binding sites that are not the case for C1/C2. Additionally reported macrocyclic compound is completely soluble in media used for the study so it is not the solid state detection and has limited applicability for its use for removing anions.
Article titled “Colorimetric sensing of anions in water using ratiometric indicator-displacement assay” by L Feng et al. published in Analytica Chimica Acta, 2012, 743, pp 1-8 reports use of colorimetric indicator-displacement assay (IDA) array for the determination of ten anions in water. The color changes in IDA array provide facile identification of these anions with no misclassification. The sensor array consists of different combinations of colorimetric indicators and metal cations. The colorimetric indicators chelate with metal cations, forming the color changes.
U.S. Pat. No. 8,008,090B2 discloses vaprochromic coordination polymers useful for analyte detection. The vapochromism may be observed by visible color changes, changes in luminescence, and/or spectroscopic changes in the infrared (IR) signature. In one particular embodiment a new class of [Metal(CN)2]-based coordination polymers with vapochromic properties is described, such as Cu[Au(CN)2]2 and Zn[Au(CN)2]2 polymers, wherein said analyte is a volatile organic compound. The compound show differential behaviour when exposed to a variety of solvent vapours e.g. DMSO, water, etc. However, they have not been explored for detection of analytes in a solution state
Article titled “Structure, Luminescence, and Vapochromism of Bridged Cationic Copper(I) Dimers and Polymers” by D E Royzman et al. published in Journal of Inorganic and Organometallic Polymers and Materials, January 2014, 24 (1), pp 66-77 reports the dimeric complex of Cu(I) [Cu2(PPh3)4(MeCN)2(Bpy)](BF4)2 (1a, Bpy=4,4′-dipyridyl) self-assembles in CH2Cl2 or acetone and shows intense photoluminescence (excitation λmax=356 nm, emission λmax=486 nm, φ=0.47).
Article titled “Colorimetric response to anions by a “robust” copper(II) complex of a [9]aneN3 pendant arm derivative: CN− and I− selective sensing” by MA Tetilla et al. published in Chem Commun (Camb), 2011, 47 (13), pp 3805-3807 report a 1:1 complex [Cu(L)](BF4)2.MeCN (1) of the tetradentate ligand 1-(2-quinolinylmethyl)-1,4,7-triazacyclononane (L) selectively changes its colour in the presence of CN− in H2O and MeCN (without undergoing decomplexation from the macrocyclic ligand). The same complex in MeCN assumes different colours in the presence of CN− or I−. However sensing in solid state and completely aqueous media (100% water) is not investigated.
Article titled “Luminescence vapochromism in solid materials based on metal complexes for detection of volatile organic compounds (VOCs)” by X Zhang et al. published in J. Mater. Chem., 2012, 22, 11427-11441 reports that vapour-triggered luminescence colour changes are mostly relevant to a variation of weak interactions such as metal-solvent bonds, metallophilic contacts, π-π stacking, hydrogen bonding, host-guest affinity or reversible isomerization etc.
It has now become essential to find highly sensitive, broad spectrum, cost-effective anion sensors which can detect wide range of anions in aqueous as well as non-aqueous media via optical detection methods. The feasibility of anions sensors as solid state detector makes it more convenient to use in the sensing devices. Solid state thin film of sensors after recognizing anions can transduce signal which can be detected by optical, electrochemical and electrical methods. Metal based anion receptors have shown enigmatic potential for their uses as anion detector. Metals can 1) act as a part of the colorimetric or luminescent reporter group; 2) they can provide template for anion induced self-assembly and 3) provide anion binding site due to Lewis acidity. Tailor made designing of structurally dynamic 1D or 2D coordination framework where metal provides site for analyte interaction which in turn offers switchable flexibility thereby rendering efficient solid state sensing method. Therefore, there is a need for designing novel coordination complexes for effective vapochromic and anion sensing property.